The invention relates to starting an internal combustion engine. In particular, the invention pertains to a fluid motor and system for starting a compression ignition or spark ignition engine.
Use of internal hydraulically or pneumatically actuated pistons as a device to start an engine requires a fast acting servo-hydraulic valve having a high fluid flow rate. Such valves are expensive and have limited long-term prospects for cost reduction.
The engine produces pressurized hydraulic or pneumatic fluid output. Because the pressure of hydraulic output is high, the ratio of combustion-piston area to the hydraulic-piston area must be large. Due to this requirement, the cross sectional area of the fluid motor pistons is relatively small. Therefore, a very high pressure in the actuating fluid motor cylinders is required to generate a sufficiently large force in order to move the pistons fast enough and produce sufficient engine cylinder pressure to start the engine. Due to the limited fluid motor piston area, the hydraulic system may not be able to provide enough force to move the engine pistons together sufficiently to produce sufficient temperature for combustion, especially when the intake air temperature is very cold.
It is necessary to use nearly the highest hydraulic pressure possible to start the engine, approximately 6000 psi. Because the pressure against which the engine output is applied is so high, upon starting, the engine must immediately fire at the highest power level and go to the equivalent of wide open throttle from a cold start. Coolant and lubricant preheating may be required to make engine starting possible under these conditions. But such preheating is impractical in a vehicle application, except in a test cell environment.
Furthermore, a relatively large, separate accumulator must be carried on the vehicle to start the engine at anytime with such fluid actuation under these starting conditions. These requirements represent important difficulties toward integrating a hydraulic drive system in a vehicle. An alternate engine starting technique is required.